The present invention relates generally to wavelength division multiplexing and, more particularly, to an improved diffraction grating for wavelength division multiplexing/demultiplexing devices.
Currently, approximately 70% of the wavelength division multiplexing (WDM) market in the United States utilize thin film filtering based multiplexing/demultiplexing devices. However, there are many difficulties associated with thin film filtering based multiplexing/demultiplexing devices. For example, for high channel count (i.e., 32 plus channels) multiplexing/demultiplexing devices, channel uniformity is important. However, due to the cascade configuration typically associated with thin film filtering based multiplexing/demultiplexing devices, channel uniformity simply cannot be maintained. That is, insertion losses will continually increase as light continues to be transmitted through each individual thin film filter.
Recently, multiplexing/demultiplexing devices incorporating array waveguide gratings (AWG""s) have emerged as an attractive alternative to thin film filtering based multiplexing/demultiplexing devices due to their integrated compact structure and high channel count capabilities. However, AWG based multiplexing/demultiplexing devices still suffer from several intrinsic disadvantages such as high insertion loss, high channel crosstalk, and active temperature control. For example, although some recent laboratory experiments on AWG based multiplexing/demultiplexing devices have achieved insertion losses as low as 0.8 dB (on the chip level), commercially available AWG based multiplexing/demultiplexing devices still only achieve insertion losses on the order of 5-6 dB.
An alternative to both thin film filtering based multiplexing/demultiplexing devices and AWG based multiplexing/demultiplexing devices are bulk diffraction grating based multiplexing/demultiplexing devices. Bulk diffraction grating based multiplexing/demultiplexing devices are founded on well known technology and have many unique advantages such as, for example, very low insertion loss and high channel count capability. However, thermal instability and narrow channel passband are two major drawbacks which prevent bulk diffraction grating based multiplexing/demultiplexing devices from prevailing over other types of multiplexing/demultiplexing devices. For example, referring to FIG. 1, there is shown a typical diffraction grating 10 that is used in a typical bulk diffraction grating based multiplexing/demultiplexing device. The diffraction grating 10 comprises a glass substrate 12, a polymer grating layer 14 formed on the glass substrate 12, and a metal coating layer 16 formed on the polymer grating layer 14. The metal coating layer 16, which is used to increase the reflectivity of the diffraction grating 10, is typically formed of gold (Au) or aluminum (Al).
Problematically, thermal expansion of the material of the glass substrate 12 can change the groove spacing of the diffraction grating 10. In addition, differences in the coefficients of thermal expansion between the materials of the glass substrate 12, the polymer grating layer 14, and the metal coating layer 16 can cause non-uniform deformations in the diffraction grating 10 when the diffraction grating 10 is subject to variations in temperature such as, for example, variations in ambient temperature, thereby distorting the profile of the diffraction grating 10. These non-uniform deformations in the diffraction grating 10 can in turn cause the efficiency of the diffraction grating 10 to decrease. For example, referring to FIGS. 2 and 3, there are shown plots derived from experimental data showing changes in diffraction beam angle (i.e., grating dispersion) and grating groove density, respectively, due to temperature variations of the diffraction grating 10.
As can be seen from FIGS. 2 and 3, bulk diffraction grating based multiplexing/demultiplexing devices are extremely temperature sensitive. The thermal effects resulting from the temperature sensitivity associated with bulk diffraction grating based multiplexing/demultiplexing devices can generally be classified into one of two categories: 1.) diffraction grating dispersion change due to expansion/contraction of the substrate material; and 2.) diffraction grating efficiency decrease due to groove profile distortion. Obviously, it would be desirable to provide a technique for minimizing, or eliminating, these thermal effects resulting from the temperature sensitivity associated with bulk diffraction grating based multiplexing/demultiplexing devices in either one or both of these two categories. More particularly, it would be desirable to provide a technique for efficiently and cost effectively minimizing, or eliminating, the above-described thermal effects resulting from the temperature sensitivity associated with bulk diffraction grating based multiplexing/demultiplexing devices.
The primary object of the present invention is to provide an improved diffraction grating for wavelength division multiplexing/demultiplexing devices.
The above-stated primary object, as well as other objects, features, and advantages, of the present invention will become readily apparent to those of ordinary skill in the art from the following summary and detailed descriptions, as well as the appended drawings. While the present invention is described below with reference to preferred embodiment(s), it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility.
According to the present invention, an improved diffraction grating for wavelength division multiplexing/demultiplexing devices is provided. The improved diffraction grating has a glass substrate, a polymer grating layer located adjacent to the glass substrate, and a metal coating layer located adjacent to the polymer grating layer. The improvement comprises a polymer coating layer located adjacent to the metal coating layer, and a glass cover located adjacent to the polymer coating layer, wherein the polymer coating layer and the glass cover compensate for thermal characteristics associated with the polymer grating layer and the glass substrate, respectively.
In accordance with other aspects of the present invention, the glass substrate and the glass cover are beneficially formed of the same material and have substantially the same thickness. Ideally, the glass substrate and the glass cover are formed of a material having a low coefficient of thermal expansion. For example, the glass substrate and the glass cover are preferably formed of a material from the group consisting of: fused SiO2; CLEARCERAM-Z, manufactured by Ohara, Inc.; ZERODUR(copyright), manufactured by Schott Glass Technologies, Inc.; ULE(copyright), manufactured by Corning, Inc.; and other glass materials having similar characteristics. Also, the glass substrate and the glass cover each ideally have a thickness range of 3 to 8 mm.
In accordance with further aspects of the present invention, the polymer grating layer and the polymer coating layer are beneficially formed of the same material and have substantially the same thickness. Ideally, the polymer grating layer and the polymer coating layer are formed of an epoxy material having at least some, and preferably all, of the characteristics of: a high Tg; a low viscosity; a low linear volume shrinkage; and a relatively high flexibility. For example, the polymer grating layer and the polymer coating layer are preferably formed of a material from the group consisting of: OG198-50 manufactured by Epoxy Technology, Inc.; OG198-53 manufactured by Epoxy Technology, Inc.; and other epoxy materials having similar characteristics. Also, the polymer grating layer and the polymer coating layer each ideally have a thickness range of 25 to 50 xcexcm.
In accordance with still further aspects of the present invention, the material of the polymer grating layer and the material of the glass cover are preferably optically matched.
According to the present invention, an athermal diffraction grating for wavelength division multiplexing/demultiplexing devices is also provided. The athermal diffraction grating comprises a glass substrate, a polymer grating layer located adjacent to the glass substrate, a metal coating layer located adjacent to the polymer grating layer, a polymer coating layer located adjacent to the metal coating layer, and a glass cover located adjacent to the polymer coating layer.
According to the present invention, a method for fabricating an improved diffraction grating for wavelength division multiplexing/demultiplexing devices is further provided. The improved diffraction grating has a glass substrate, a polymer grating layer located adjacent to the glass substrate, and a metal coating layer located adjacent to the polymer grating layer. The method comprises forming a polymer coating layer adjacent to the metal coating layer, and then forming a glass cover adjacent to the polymer coating layer, wherein the polymer coating layer and the glass cover compensate for thermal characteristics associated with the polymer grating layer and the glass substrate, respectively.